Process for producing photographic images with photosensitive materials and products produced thereby

ABSTRACT

THIS INVENTION RELATES TO A PROCESS OF PRODUCING PHOTOGRAPHIC IMAGES USING PHOTOSENSITIVE MATERIALS WHICH HAVE BEEN PRESENSITIZED BY EXPOSURE TO ACTIVATING RADIATION SO THAT THE PHOTOSENSITIVE SUBSTRATE BECOMES SENSITIVE TO WAVELENGTHS OF RADIATION TO WHICH THE SUBSTRATE IS USUALLY NOT SENSITIVE. THE PHOTOSENSITIVE MATERIAL WHICH BECOMES REVERSIBLY ACTIVATED UPON EXPOSURE TO ACTIVATING RADIATION BUT PREFERABLY REMAINS UNCHANGED AFTER SUCH EXPOSURE IS HEREINAFTER REFERRED TO AS A PHOTOCONDUCTOR. THE PRESENSITIZED PHOTOCONDUCTOR IS EXPOSED TO A PATTERN OF LIGHT TO WHICH THE PRESENSITIZED PHOTOCONDUCTOR IS SENSITIZED TO FORM A LATENT IMAGE WHICH IS DEVELOPED BY KNOWN METHODS, E.G. USING LIQUID REDOX SYSTEMS TO OBTAIN THE VISIBLE IMAGE OF THE PATTERN. THE REDOX SYSTEM MAY BE APPLIED TO THE PHOTOCONDUCTOR EITHER BEFORE OR AFTER THE STEP OF LATENT IMAGE FORMATION. IT HAS ALSO BEEN FOUND THAT THE QUALITY OF THE VISIBLE IMAGE MAY BE SUBSTANTIALLY IMPROVED BY SELECTION OF LIGHT OF SPECIFIC WAVELENGTH TO ENSURE OPTIMUM DENSITY DIFFERENTIALS BETWEEN THE BACKGROUND AND THE VISIBLE IMAGE.

Jan. 12,1971

G. M. FLETCHER ETA!- PROCESS FORPRODUCING PHOTOGRAPHIC IMAGES WITHPHOTOSENSITIVE MATERIALS AND PRODUCTS PRODUCED THEREBY Filed July 13,1967 I I, I

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BY ma AGENT.

United States Patent 3,554,749 PROCESS FOR PRODUCING PHOTOGRAPHIC IMAGESWITH PHOTOSENSITIVE MATERIALS AND PRODUCTS PRODUCED THEREBY GeraldMatthew Fletcher, Arlington, Amal Kumar Ghosh,

Lexington, and Fahd G. Wakin, Bedford, Mass., assignors to ItekCorporation, Lexington, Mass., a corporation of Delaware Filed July 13,1967, Ser. No. 653,198 Int. Cl. G03c 5/04 U.S. Cl. 96-27 27 ClaimsABSTRACT OF THE DISCLOSURE This invention relates to a process ofproducing photographic images using photosensitive materials which havebeen presensitized by exposure to activating radiation so that thephotosensitive substrate becomes sensitive to wavelengths of radiationto which the substrate is usually not sensitive. The photosensitivematerial which becomes reversibly activated upon exposure to activatingradiation but preferably remains unchanged after such exposure ishereinafter referred to as a photoconductor. The presensitizedphotoconductor is exposed to a pattern of light to which thepresensitized photoconductor is sensitized to form a latent image whichis developed by known methods, e.g. using liquid redox systems, toobtain the visible image of the pattern. The redox system may be appliedto the photoconductor either before or after the step of latent imageformation. It has also been found that the quality of the visible imagemay be substantially improved by selection of light of specificwavelength to ensure optimum density differentials :between thebackground and the visible image.

BACKGROUND OF THE INVENTION (a) Field of invention This inventionrelates to processes for production of photographic images usingpresensitized light-sensitive compositions and products obtainedthereby.

(b) Description of the prior art The use of media comprisingphotoconductors for the production of latent images is described inBritish Pat. 1,043,250. In the patent, the method generally requires theformation on the media of a latent reversible image corresponding to apattern of activating light and, at a subsequent time, conversion of thelatent image to a visible image using liquid redox system which contactat least the light-activated portions of the media to form a deposit onthe media corresponding to the light-activated portions. The product ofthis aspect of the described process is a negative corresponding to theinitial pattern of activating light, which may be used to produce apositive in known manner. A further modification of the describedprocess which results in a positive visible image involves the furtherstep of deactivating those portions of the media which were activated bythe initial pattern, after which development leads to the positive.

In the art of electrostatic printing, as described for example in US.Pats. 3,041,168 and 3,121,006, the basic principle involves providing asubstantially uniform electrostatic charge on the photoconductivecoating of the recording element followed by discharge of selected areasin order to produce an electrostatic image, which is then developedusing powdered developing material, which process requires specialdeveloper materials and costly recording media.

SUMMARY OF THE INVENTION It has now been unexpectedly found that latentimages may be produced utilizing presensitized media comprising aphotoconductor. The latent image is produced by exposing such apresensitized medium to a pattern of light to which the medium issensitive to obtain a latent image corresponding to the pattern. Thelatent image is then developed, preferably substantially immediatelyafter formation for optimum results. The developer is preferably aliquid redox system which on contact with activated portions of themedium deposits a solid, visible residue on the activated portions. Ifdesired, the redox system may be applied to the medium prior to thelatent image formation step in which case the visible image is obtainedwhen the medium is still wet. On the other hand, if the redox system isapplied to the medium and the medium then dried prior to latent imageformation, the visible image is obtained by immersion of the medium in asuitable solvent system.

Optimum results are obtained when latent image formation is conductedwith light of wavelength to provide maximum density dilference betweenthe visible image and the background of the medium. The determination ofoptimum wavelengths of light for this purpose is readily accomplished bydetermination of the spectral response of the medium before and afterpresensitization, as hereinafter described.

DESCRIPTION OF PREFERRED EMBODIMENTS The photoconductor or photocatalystis not limited to any group of compounds but may include both organicand inorganic photosensitive materials. Preferred photoconductors usefulin this invention are metal containing photoconductors. A preferredgroup of such photosensitive materials are the inorganic materials suchas com pounds of a metal and a nonmetallic element of group VIA of theperiodic table 1 such as metal oxides, such as zinc oxide, titaniumdioxide, antimony trioxide, aluminum oxide, zirconium dioxide, germaniumdioxide, indium trioxide, chromium oxide, magnesium oxide, cerium oxide,hydrated potassium aluminum silicate tin oxide (SnO bismuth oxide (Bi Olead oxide (PbO), beryllium oxide (BeO), silicon dioxide (SiO- bariumtitanate (BaTiO tantalum oxide (Ta O tellurium oxide (TeO and boronoxide (B 0 metal sulfides such as cadmium sulfide (CdS), zinc sulfide(ZnS) I and tin disulfide (SnS metal selenides such-as cadmium selenide(CdSe). Metal oxides are especially preferred photoconductors of thisgroup. Titanium dioxide is a preferred metal oxide because of itsunexpectedly good results.

Also useful in this invention as photoconductors are certain flourescentmaterials. Such materials include, for example, compounds such as silveractivated zinc sulfide, zinc activated zinc oxide, manganese activatedzinc phosphate Zn (PO an admixture of copper sulfide, antimony sulfide(SbS) and magnesium oxide (MgO), and cadmium borate.

Organic photoconductors suitable for use in this invention are, forexample, the imidazolidinones, the imidazolidinethiones, thetetraarylazacyclooctatetraenes, and thiazines, such as 1,3-diphenyl 4,5bis(p-methoxyphenyl) imidazolidinone-Z;4,5-bis(para-methoxyphenyl)imidazolidinone-2; 4 phenyl 5(paradimethylaminophenyl) imidazolidinone-2;4,5-bis(para-methoxyphenyl)imidazo- Periodic "table from Langes Handbookof Chemistry, 9th edition, pp. 56-57, 1956.

lidenthione-2; 3,4,7,8-tetraphenyl l,2,5,6tetraazacyclooctatetraene-2,3,6,8; and methylene blue.

Also useful as photoconductors in this invention are the heteropolyacidssuch as phosphotungstic acid, phosphosilicic acid, and phosphomolybdicacid.

While the exact mechanism of the present process is not known, it isbeileved that the presensitization, i.e. exposure to activating light,e.g. ultraviolet light, causes the transference of electrons of thephotoconductor from the valence band to the conductance band, or atleast to some similar excited states wherein the electron is looselyheld thereby converting the photoconductor from an inactive to an activeform. If the photoconductor in the active form is in the presence of anelectron-accepting agent, a transfer of electrons will take placebetween the photoconductor and the electron-accepting agent and thelatter will be reduced. Accordingly, a simple test to determine whetherphotoconductors have reducing properties is to mix the material inquestion with aqueous silver nitrate. In the absence of light, little,if any, reduction of silver ions should occur. At the same time asexposing the same mixture to light, a control sample of an aqueoussilver nitrate solution alone is similarly exposed and if the mixturedarkens faster than the control sample, the test material is aphotoconductor with reducing properties.

It is evident that the gap between the valence and the conducting bandof a compound determines the energy needed to make electron transitionsand the light required to provide the needed energy is called bandgaplight, as employed herein. The higher energy needed, the higher thefrequency to which the photoconductor will respond. It is known in theart that electrons may be present in secondary levels within the bandgap due to impurities on defects in the structure of the photoconductor.With light of suitable energy, which in this case would be less than theband gap, electrons from these levels could be raised to the conductionband. A typical example of a secondary level due to a defect in thestructure would be an F-center (electrons trapped at negative ionvacancies in an alkali halide crystal). The band gap of KCl is about 8.5ev. (1460 A.), but the secondary levels due to F-centers are about 2.4ev (5400 A.) below the conduction band. Electrons could be raised to theconduction band with 5400 A. light. An example of an impurityphotoconductivity could be ZnS doped with Cu. The band gap of ZnS isabout 3.7 ev. (3350 A.), but by doping it with Cu one could introducesome secondary levels which would result in photoconduction due to 4600A. light.

As is generally known, the activation of photoconductors, i.e.transference of electrons from valence bands to conductance bands, isnot permanent but rather the activation decays primarily as a functionof time. The decay is apparently due to the loss of electrons in theconductance bands, the electrons reverting to lower energy levels, manyreverting to the original valence band and others to energy levelsintermediate between the respective bands, i.e. secondary levels, ortraps. After decay of the activated photoconductor, the medium retainslittle, if any, ability to reduce silver ions, or similar metal ion, dueto the fact that there are little, if any, electrons in the conductanceband. When reference is made herein to a decayed sensitized medium it isintended that the photoconductor is in a state intermediate between theactive and inactive states by virtue of the fact that electrons of thephotoconductor are in the secondary levels, or traps.

When the decayed sensitized medium is exposed to an image pattern oflight of wavelength longer than the bandgap light, the energy providedis sufficient to raise the electrons in the secondary levels to theconductance band, but not sufiicient to raise electrons from the valenceband to the conductance band. This results in a latent image on themedium corresponding to the pattern, and when the medium is brought intocontact with an electron acceptor, electron transfer occurs.Accordingly, if the medium is contacted with a liquid re dox system,reduction of the reducible component thereof occurs. If the reduciblecomponent, in the reduced form, is a particulate solid, the resultobtained is a visible image corresponding to the pattern.

The foregoing theoretical explanation is offered to enable a betterunderstanding of the present invention and is believed to reasonablyinterpret the phenomenon of this invention. Of course, the applicantsare not necessarily bound by this explanation.

In the sensitization of the photoconductor it is preferred to utilizehigh exposure energies of bandgap light. For example, the exposureenergy should be about 10 millijoules/cm. or greater for best results,while energies ranging from 0.05 millijoule/cm. are found operable.

For decaying the photoconductor sensitized with bandgap light it ispreferred to allow the medium to stand over a period of time usually ofat least about one hour to ensure substantially complete decay. Asmentioned hereinbefore, samples of the sensitized medium may be testedusing, for example, aqueous silver salt solutions,

' to determine the time required for decay which will of course bedependent on the photoconductor, the exposure energy, and other factorsknown to those skilled in the art. After sensitization, the mediumshould be protected from exposure to activating light prior to exposureto a light pattern to form the latent image and subsequent to this stepuntil the medium is developed. Techniques for avoiding unintendedexposure are well known and need not be enumerated for those skilled inthe art.

The exposure of the decayed sensitized photoconductor to the imagepattern may be conducted using standard techniques. The light utilizedis preferably visible light, of wavelength longer than the bandgap lightof the photoconductor. Such wavelength preferably ranges from about 4200A. to about 7000 A. and may even include the near infrared. Certainranges are more effective depending on the photoconductor, among otherfactors, and a minimum of testing will indicate the optimum range forthe specific photoconductor. For example, a range from about 4500 A. toabout 6000 A. gives excellent results when titanium dioxide is thephotoconductor. The time of exposure may be varied considerably, fromfractions of a second to several minutes without appreciable variationin the results.

As should be obvious to those skilled in the art, the latent image atthis stage may be stored of course protected from unintentional lightactivation, but care should be taken to avoid substantial decay of thelatent image for which reason long periods of storage should be avoided.As desired, the visible image may be produced, or alternatively,modification of the latent image may be effected, e.g. by erasure, inwhole or in part, of the latent image and/or super-imposing of a furtherimage. Such erasure may be accomplished by reactivating thephotoconductor with light of wavelength longer than bandgap light.

When it is desired to convert the latent image to a visible image, themedium is preferably immediately developed. Immediate development, whilenot necessary, minimizes the effect or decay of activation, for whichreason it is preferred. In this respect, it may be advisable, but notnecessary, to apply the developer to the medium prior to the latentimage formation step and the visible image will form on latent imageformation. If the developer requires solvent, the visible image will notform unless solvent is present. Therefore, if the developer includingsolvent is applied to the medium and the solvent is subsequently removedprior to latent image formation exposure of the treated medium to thesolvent will result in visible image formation.

As indicated herein, the developer may be applied immediately after thelatent image formation or at least within a reasonable period of timebefore appreciable decay of the activation as will be appreciated bythose skilled in the art.

The developing agents preferred for this invention are liquid redoxsystems preferably comprising heavy metal ions such as silver, gold,copper, mercury, and other noble metal ions. British Pat. 1,043,250fully describes the intended developing agents and processes fordeveloping and fixing for use in this invention and is incorporatedherein by reference.

As mentioned hereinbefore, optimum density difference between thevisible image and the background of the medium is attainable byselection of light of specific wavelengths. The determination of theoptimum wavelength is readily accomplished by a mere comparison of thespectral response of the photoconductor before and after bandgap lightradiation. For example, the spectral response curve of normal titaniumdioxide is determined by plotting the activation versus the wavelengthof light, the activation being measured by the ability of thelightactivated photoconductor to reduce silver ions from solution asindicated by the density above fog on the thustreated medium. If thedensity above fog is plotted against the wavelength of light, theresulting curve approaches zero density as the wavelength approachesthat of visible light.

When the titanium dioxide is sensitized with bandgap light, decayed andexposed to light of longer wavelength than bandgap light at the sameexposure energy, the corresponding curve does not approach zero in thevisible light region of the curve.

Typical'spectral response curves are presented in FIG. I whichrepresents the spectral response of titanium dioxide both before andafter ultraviolet sensitization and decay.

Curve A represents the spectral response of titanium dioxide (coated inthin layer on a sheet of paper) after exposure to light of wavelength3660 A. at an intensity of 36.6 .watts/cm. for five minutes, after await time of 1.5 hours (to decay the activation). Samples of the mediumcomprising titanium dioxide so treated were then exposed to light ofvarying wavelength of exposure energy=15.3 joules/cm. and the densityabove fog measured for each sample after treatment with alcoholic silvernitrate. The curve was plotted on the basis of the density above fogcorresponding to the wavelength of light applied.

Curve B represents the normal spectral response of identically treatedsamples of titanium dioxide sheets with ultraviolet sensitizationomitted.

It is obvious from a comparison of the respective curves that light offrom about 4500 A. and even lower to 4200 A. could be used to obtaindensity differences between the image and the background of the mediumto obtain a visible image. It is obvious too that with light ofwavelength lower than about -5000 A., the background would be of varyingshades of gray but the image would be darker by comparison. Above 5000A., the background should be relatively free of silver deposits whilethe image is of considerable density. Choice of optimum wavelength forthe exposure to visible light becomes quite obvious and is determined bythe desired end result. For absolute image clarity (black on white ornear-white), wavelengths in the vicinity of 5000 A. and higher should beemployed. Where such considerations are not of paramount interest, anywavelength ranging from 4200 A., up

to about 7000 A. could be employed, as practicality dictates.

When visible light of higher exposure energy is utilized,

the spectral response curve shifts to the higher wavelengthsas isevidenced by curves A and B which are determined in identical manner ascurves A and B with the exception ferences are substantial over a longerand higher range of wavelength values. Optimum wavelengths of visiblelight should be quite apparent in view of the foregoing comments.

For the purpose of the foregoing discussion the curves of the graphrepresent the image areas and the background of the medium. Curve Acorresponds to the area activated by visible light, i.e. the image whileCurve B corresponds to the background, which is unaffected by theexposure to visible light to form the latent image.

In the initial sensitizing of the photoconductor, bandgap light isemployed. Exemplary photoconductors with corresponding bandgap andabsorption edges are listed in Table 1.

The initial sensitizing of the photoconductor may also be augmented bythe presence of dyes in the photoconductor medium. As is well known, thesensitivity of the semiconductor may be increased by known sensitizationtechniques such as admixtures of dyes with the photoconductor, Dyesensitizing permits the use of light of wavelength longer than thebandgap light of the photoconductor.

When employed as data storage media according to the present invention,the photoconductor materials previously discussed herein can suitably beemployed in bulk, e.g., in the form of a continuous layer. When used inimage forming processes, the photoconductors are conveniently applied toa suitable backing which may be either porous or non-porous, such as ofpaper, wood, aluminum, glass and the like. The photoconductor, which issuitably used in the form of finely divided particles, may simply bedeposited on the surface of such a backing, or can be deposited on sucha backing in a hydrophobic or, preferably, a hydrophilic binder known tothose skilled in the art of making radiation sensitive papers. Suitablehydrophobic binders, for example, include the polyvinylacetate resinbinders commonly used in the preparation of papers for electrostaticprinting processes. Typical of the preferred hydrophilic binders havinga limited water solubility are gelatin, polyvinyl alcohol, and ethylcellulose. for example, though many other materials of both types couldbe mentioned. Particularly advantageous results are employed when thefinely divided photoconductor is merely dispersed in the interstices ofa fibrous backing such as paper, the fibers of the backing acting tolock in and to hold the photoconductor particles in the finishedstructure. For example, the photoconductor is easily incorporated inpaper during. its manufacture by methods known in the papermaking art.

A better understanding of the invention will be had by reference to thefollowing examples, given by way of illustration of the methods ofcarrying out the invention.

EXAMPLE 1 A mixture of 4 parts by weight of titanium dioxide and 1 partby Weight of an emulsion of polyvinyl alcohol resin containing about 50percent solids in water is used to coat paper sheets.

A sheet of the coated paper is exposed to light (ultraviolet) ofwavelength 3660 A. for five minutes at an intensity of 50 ,uwatts/cm.The so-treated paper is allowed to decay by dark storage for 30 minutes,after which it is exposed to an image pattern of activating light ofwavelength 4800 A. for five minutes at an intensity of 50 ,u.watts/cm.The latent image pattern is developed by dipping in a saturated solutionof silver nitrate in methanol followed by dipping into a solutioncomprising g. phenidone, 40 g. of citric acid monohydrate and one literof methanol. The resulting visible image bearing print is a negative ofthe latent image pattern which is fixed by immersion in an aqueoussolution of sodium thiosulfate followed by washing in running water.

EXAMPLE 2 The procedure of Example 1 is repeated with the exception thatthe latent image formation is conducted with light of wavelength 5700 A.at the same intensity and for the same time of exposure, with similarresults.

EXAMPLE 3 A sheet of the coated paper described in Example 1 is exposedto light of wavelength 3660 A. at an intensity of 50 watts/cm. for 5minutes and allowed to decay by dark storage for 30 minutes.

The sheet is exposed to an image pattern of light of wavelength 6000 A.at an intensity of 83.3 ,uwatts/crn. for a period of 5 minutes to form alatent image of the pattern. Developing with silver nitrate as describedin Example 1 gives a negative visible image corresponding to the imagepattern.

EXAMPLE 4 The procedure of Example 1 is repeated using sheets preparedwith an acrylate binder in lieu of polyvinyl alcohol with similarresults.

EXAMPLE 5 A mixture of 4 parts by weight of titanium dioxide and onepart by weight of an emulsion of polyvinyl alcohol resin containingabout 50 percent solids in water is used to coat paper sheets.

A sheet of the coated paper is exposed to light of wavelength 3660 A. atan intensity of 79 watts/cm. for 5 minutes. The so-sensitized sheet isallowed to stand in dark storage for 1.5 hours after which theactivation is substantially completed decayed. The decayed medium isthen exposed to an image pattern of light of wavelength 5700 A. atintensity of 79 watts/cm. for a period of 5 minutes. The exposed sheetis developed by dipping into a saturated solution of silver nitrate inmethanol and then in a solution of 5 g. of phenidone and 40 g. of citricacid monohydrate on one liter of methanol. A visible negative image ofthe positive exposure image is obtained. The visible image-bearin gprint is then immersed in an aqueous solution of sodium thiosulfate andfinally washed with running water.

EXAMPLE 6 The procedure of Example 5 is repeated using light ofwavelength 6900 A. at an intensity of 316 ,uwatts/cm. in the imageformation step with similar results.

The following examples illustrate the effectiveness of the presentprocess in sensitizing photoconductors to visible light in contrast withmedia comprising photoconductors sensitized with dyes to visible light.

EXAMPLE 7 A titanium dioxide paper, prepared as in Example 1, is exposedto a mercury zenon lamp for 5 minutes at an intensity in the ultravioletof approximately 20,000 ,uwatts/cm.

The thus-sensitized paper is stored for seven days and then samples areexposed to visible light for 5 sec. at all wavelengths longer than 4100A. at the intensity given. Similarly a dyed paper containing titaniumdioxide (D- 96 is similarly exposed to the same light at the sameintensities with the following results:

1 2-p-dimethylarnlnostyrylt-methylthiazole methoehloride.

The procedure of Example 7 is repeated with the exception that the waittime after sensitizing the photoconductor is eight days and the exposureto visible light is for 15 seconds to all wavelengths longer than 4650A. at the intensity given in the following results:

Density above fog Sensitized Dyed undyed paper paper Integgity(nwatts/cmfi):

EXAMPLE 9 The procedure of Example 8 is repeated employing allwavelengths of light longer than 4950 A. for 5 seconds at the intensitygiven in the following results:

Density above fog Sensitizetl Dyed undyed paper paper EXAMPLE 10 Theprocedure of Example 8 is repeated with the exception that the wait timeafter sensitizing is 16 days with almost identical results asillustrated in the following results:

Density above fog Sensitized Dyed undyed paper paper The sensitizingeffect is thus illustrated to be a longlasting effect, which is quitestable.

The developer solution employed in the foregoing examples containssilver ion which is preferred. In practice, the developer may includeany metallic ion which is at least as strong an oxidizing agent as ioniccopper, e.g. gold, mercury, platinum, lead and copper.

In lieu of developing with redox system one may employ resins which areaffected by the light activated areas of the photoconductor medium toproduce relief images. For example, a resin coating comprised of 15parts acrylamide to 1 part methylene-bis-acrylamide will be renderedinsoluble to water and the soluble resin may be removed by water-washingthe medium leaving the background of the medium free of resin while theimage areas retain the insolubilized resin, resulting in both a visibleand a relief image.

In general, any image-producing agent may be employed to correct thelatent image into a visible image. For example, solid toners may beemployed as described in British specification No. 935,621. In addition,visible images may be produced using charged particles as employed inxerographic developing. As is appreciated by those skilled in the art,the selection of suitable imageproducing agents is predicated on theactivation of the photoconductor. I

If desired, the image-producing agent may be applied to the medium afterdecay of the initial sensitization, and prior to image exposure asmentioned hereinbefore.

The present process also permits use of dyes to sensitize thephotoconductor to additional ranges of electromagnetic radiation. Suchdyes are well known to the art and include, for example, cyanine dyes,dicarbocyanine dyes, the carbocyanine dyes, and hemicyanine dyesr Afterthe sensitization of the photoconductor, image formation anddevelopment, the dye may be removed by dissolving the dye out of thesubstrate or by contacting with a suitable oxidizing agent. Preferably,the dye may be removed by contacting the medium with a solution of athionate, e.g. sulfites and/or bisulfites, preferably in'the form ofsalts with alkali or alkaline earth metals. The preferred method ofbleaching with thionates is described in commonly-owned copendingapplication U.S. Ser. No. 641,126 filed May 25, 1967, the disclosure ofwhich is incorporated herein byreference.

In the step of sensitizing-the medium comprising a photoconductor, theuse of bandgap light for this purpose has been described as a preferredmethod. In addition to bandgap light the medium comprising aphotoconductor may also be sensitized by use of gamma rays or X-rays,neutrons and/ or heat in lieu of bandgap light. The medium aftersensitizing by these additional methods of activation is then allowed todecay and is useful in the same manner in producing images as hereindescribed.

In the image-forming step, customary methods such as projection orcontact printing may be utilized. A master is either projected on themedium or printed by contact with the medium using art-recognizedprocedures.

Since the present invention permits recording of images on recordingmedia by use of ordinary light, i.e. visible light, it provides arelatively simple and economical method. A further advantage of thepresent process resides in the ease of erasure for correction, e.g. byexposure of selected areas of the latent image to high intensity ofbandgap light of the photoconductor or by any of the art recognizedprocedures such as application of heat to the areas selected forerasure. Over printing may then be accomplished for effecting correctionof the stored images. In actual use, the recording medium would be inthe form of a roll of tape or film.

The latent images produced in accordance with the present invention, ordevelopment, yield negative visible images corresponding to the originalpattern of light. Such negative visible images may be used in formingpositive images used standard techniques known in the photographic art.

The latent images may also be used to produce positive visible imageswithout being developed first as negatives. As previously mentioned, thelatent images tend to decay as a function of time. The decay of thelatent image may be facilitated by heat treatment using temperatures upto about 250 F. The decayed latent image, on flooding of thephotoconductor medium with visible light preferably light of the samewavelength as that used in the latent image formation becomes a positivelatent image, which on development e.g. with liquid redox systems, givesa visible positive image corresponding to the latent negative image.This procedure is described in commonly owned application U.S. Ser. No.653,148, concurrently filed herewith.

What is claimed is:

1. A method of producing a latent image comprising the steps of:

(a) exposing to activating radiation a medium comprising aphotoconductor to thereby activate exposed portions to form a latentimage capable of reducing metal ions thereon corresponding to theexposed portion of the medium;

(b) decaying the latent image so formed so that the photoconductor is ina state intermediate between the active and inactive states in that thephotoconductor on contact with a solution of metal ions in the absenceof activating radiation is substantially non-reducing; and

(c) exposing the previously activated medium to light of wavelengthlonger than bandgap light for a time and intensity suflicient toactivate the photoconductor so exposed such that said exposedphotoconductor on contact with a solution of metal ions in the absenceof activating radiation is substantially reducing.

2. A method as in claim 1 wherein the exposure of step (a) is asubstantially uniform exposure and wherein the exposure of step (c) isan image-wise exposure.

3. A method according to claim 1 including the further step of producinga visible image by contacting the medium with an image-producing agent.

4. A method according to claim 3 wherein the imageproducing agentcomprises one which undergoes an oxidation/ reduction type reaction uponcontact with an activated photoconductor.

5. A method according to claim 4 wherein the medium is contacted withthe image producing agent prior to exposure to said light of longerwavelength.

6. A method according to claim 4 wherein the medium 1s contacted withthe image producing agent subsequent to exposure to said light of longerwavelength.

7. A method according to claim 1 wherein the photoconductor is acompound of a metal with a non-metallic element of Group VI-A of thePeriodic Table.

8. A method according to claim 7 wherein the photoconductor is selectedfrom the group consisting of titanium dioxide, zinc oxide, zirconiumdioxide, alumnum oxide, chromium oxide, magnesium oxide, indium trioxideand cerium dioxide.

9. A method according to claim 1 wherein the photoconductor is titaniumdioxide.

10. A method according to claim 2 wherein the light of wavelength longerthan said bandgap light is of a wavelength which provides optimumdensity difierence bet-ween the visible image and the background of themedium.

11. A method according to claim 2 wherein the initial exposure is to thebandgap light of the photoconductor, or gamma or X-ray or neutronirradiation. A method of producing a latent image comprising: nltlallyexposing and decaying so that the photoconductor 1s 1n a stateintermediate between the active and inactive states in that conductanceband electrons of the photoconductor are in secondary energy levels acopy medium comprrsmg titanium dioxide to thereby produce a decayed copymedium sensitive to light of Wavelength longer than the bandgap light ofthe titanium dioxide and then exposmg the decayed copy medium to apattern of light of wavelength longer than the bandgap light of titaniumdioxide to form a latent image thereon corresponding to said pat tern.

13. A method according to claim 12 including the further step ofproducing a visible image by contacting the medium with an imageproducing agent.

14. A method according to claim 13 wherein the imageproducing agentcomprises a solution of metallic ions.

15. A method according to claim 14 wherein the medium is contacted withthe image-producing agent prior to exposure to said light of longerwavelength.

16. A method according to claim 14 wherein the medium is contacted withthe image-producing agent subsequent to exposure to said light of longerwavelength.

17. A method according to claim 12 wherein the light of longerwavelength is of a wavelength which provides optimum density differencebetween the visible image and the background of the medium.

18. A method according to claim 12 wherein the light of longerwavelength is of a wavelength ranging from about 4200 A. to about 7000A.

19. A method according to claim 17 wherein the light of longerwavelength is of a wavelength ranging from about 4500 A. to about 6000A.

20. A method according to claim 14 wherein the metallic ions are atleast as strong an oxidizing agent as ionic copper.

21. A method according to claim 20 wherein the metallic ions are silverions.

22. A method according to claim 12 wherein the initial exposure is tothe bandgap light of titanium dioxide, gamma or X-ray or neutronirradiation.

23. A method according to claim 2 wherein the photoconductor is acompound of a metal with a non-metallic element of Group VI-A of thePeriodic Table.

24. A method according to claim 1 wherein the photoconductor is acompound of a metal with a non-metallic element of Group VI-A of thePeriodic Table.

25. A method according to claim 1 wherein the photoconductor is acompound of a metal with a non-metallic element of Group VI-A of thePeriodic Table.

26. A method of producing a photosensitive medium comprising titaniumdioxide which comprises the steps of exposing said medium to the bandgaplight of titanium dioxide wherein the exposure energy is at least about10 millijoules/cm. of surface of said medium and decaying the activationso produced so that the photoconductor is in a state intermediatebetween the active and inactive states in that conductance bandelectrons of the photoconductor are in secondary energy levels, andsubsequently exposing said previously activated medium to light of awavelength longer than bandgap light.

27. A method according to claim 26 wherein the exposure energy ranges upto about 20,000 milliwatts per square centimeter of surface of saidmedium.

References Cited UNITED STATES PATENTS 3,152,903 10/1964 Shepard et al.9664 3,380,823 4/1968 Gold 9627 3,414,410 12/1968 Bartlett et al 961XFOREIGN PATENTS 1,043,250 9/ 1966 Great Britain 961 OTHER REFERENCESShattuck et a1., 'Postexposure of Latent Electrostatic Images, IBM Tech.Discl., vol. 8, No. 4, September 1965, p. 529.

GEORGE F. LESMES, Primary Examiner 'R. E. MARTIN, Assistant Examiner US.Cl. X.R. 961; 25065.1

